1. Field of the Invention
The present invention relates in general to a high-speed digital data communication technology, and in more particular to broadband integrated services digital network (B-ISDN) systems wherein a plurality of data terminals such as work stations are capable of communicate with any one of the data terminals through data links associated therewith. The invention also relates to a connection-oriented local area network (LAN) employing an asynchronous data transfer mode.
2. Description of the Related Art
With the increasing needs for large transfer capacity and high speed transmission of digital data communication network systems (such as B-ISDN), the development of a more efficient data-transfer routing scheme for such network systems has been demanded strongly. In recent years, as one of the B-ISDN systems satisfying the requirements, attention is paid to a specific digital data network system that employs a specific packet-oriented data-transfer mode which uses asynchronous time division multiplexing techniques, which is called the "asynchronous transfer mode (ATM)". ATM is one of the most promising transfer modes implementing B-ISDN. A local area network using ATM is generally known as the "ATM-LAN" among those skilled in the art of digital data communication network systems.
The details of the data transmission principles in the ATM communication network system have been described in several recommendations as published by the international telegraph and telephone consultative committee (CCITT). Principally, the presently available ATM network includes data links for interconnecting a plurality of network switches. A plurality of terminals (such as work stations, personal computers, etc.) are connected to the network switches by way of links associated therewith. The links have an input port and an output port with respect to each terminal. When a certain terminal communicates with another terminal, the former may be called the "source terminal" or "sender", whereas the latter is called the "destination or target terminal" or "receiver". A routing for enabling the data transfer between them is selected and determined using identifier(s).
More specifically, with the ATM network, the multiplexed information flow to be communicated between the "sender" and "receiver" terminals is organized into a plurality of blocks or packets of fixed size, that is, packets. These packets are generally called the "cells" in the digital data communication network art. A cell consists of an information field called the "payload" and a header section. The information field stores therein information to be transferred; the header is arranged to store a variety of kinds of parameters required to perform the transfer of information. The information may be disassembled into a plurality of cells, each of which has a fixed length of 53 bytes, for example; in such circumstances, the information field is assigned with 48 bytes, while the header is assigned with 5 bytes. The parameters include a 8-bit or 12-bit virtual path identifier (VPI) and a 2-byte virtual channel identifier (VCI). The recommended cell header format and structure is disclosed, for example, in the international telegraph and telephone consultative committee (CCITT) integrated services digital network (ISDN), Recommendation I.361, "B-ISDN ATM Layer Specification", Geneva, 1991, at pp. 1-2 (FIGS. 1-2).
The primary role of the header is to identify certain cells that belong to the same virtual channel within the asynchronous time-division multiplex. The transfer capability may be assigned by negotiations, depending upon the source requirements and the available capacity. The network identifies a connection by making reference to the VCI and VPI. In other words, the connection for enabling the data transmission in the ATM network is supported by the contents of the cell header. In this respect, the ATM may also be regarded as a connection-oriented data-transmission technique.
In the existing ATM scheme, the cell header parameters such as VCI and VPI are unique or remain uniform with respect to only one of the switches or cell exchange nodes (which may also be called the physical interface point in some cases) internally arranged in the ATM network. The VCI/VPI parameters do never remain uniform in value throughout the entire network. It will unconditionally happen that the same VCI/VPI parameter values may coexist at different interface points within the ATM network. Looking at the ATM network as a whole, in case where data packets or cells are transferred from a sender toward a receiver in accordance with a presently selected connection routing in a session, the VCI/VPI parameters are converted or rewritten into different values every time such parameters pass through a different one of the cell exchange nodes included in the selected route in such a manner that a rewritten value of each parameter matches a data link associated with such node. At each cell exchange node, the transfer of an outgoing cell is carried out by searching for a routing table prestored therein with the VCI/VPI values of an incoming cell being as a key, and by adding to the cell a routing tag that is effective only at a corresponding cell exchange node. The routing table information for enabling the VCI/VPI "translation" and the addition of routing tag at each cell exchange node is written when respective connection is set in the ATM network. When the connection regarding the present session is terminated, a corresponding entry information will be deleted.
Another saying of this is that the prior art ATM network system employs the network node interface (NNI) scheme to interconnect network segments, which may be switch nodes or sub-networks included in the system. In the case of performing an ATM session, the terminal requests a connection setting to the network. Upon the receipt of the connection setting request, the network makes a communication reservation and determines a routing necessary to transfer packets of information (or cells) from the sender (or source terminal) to a receiver (or destination terminal). The VCI/VPI information indicative of the routing thus determined is notified to the terminal. Such VCI/VPI values are assigned to the respective switch nodes distinctly and independently. In principle, different VCI/VPI values are assigned to different nodes; in practice, assignment of identical values to different nodes may occur due to the "independency". Therefore, it should be necessary that, in data communications, the VCI/VPI values must be rewritten or converted to match each corresponding node every time the transfer cells pass through each of several nodes included in the determined routing.
It has been described that, with the ATM routing scheme, the values of VCI/VPI parameters given by the network to the switch nodes concerned are uniform only for respective interface points, i.e., data-exchange nodes (or switches). Therefore, VCI/VPI acquisition does not come without the settings of connection. The connection settings are attained by making use of a special purpose channel called a "metasignaling channel". The connection-setting procedure is performed by a connection setting server as part of the function assigned to the ATM network.
A problem of the "switch nodes dependent uniformity" arrangement of the VCI/VPI in the conventional ATM network is that the certainty and reliability of data communications may depend on the network connection setting server. Unfortunately, if the network connection setting server operates improperly (due to the occurrence of a failure, for example), all the communications in this network become unavailable. This is based on the fact that, in the prior art, the start of actual communication in each of all the communication sessions is impossible without the presence of intervening connection-setting procedure, and that, if the server malfunctions, the ATM network becomes in the communication impossible state as a whole.
Another problem of the conventional ATM network with the "nodes dependent VCI/VPI uniformity" arrangement is as follows: in a given session, when packet data is to be transferred from a source terminal to a destination terminal through cell exchange nodes selected in accordance with the presently determined routing, the VCI/VPI values must be converted every time they pass through each of such nodes. To achieve the rewrite function of the VCI/VPI values at every switch node, an internal memory of large capacity for storing a VCI/VPI conversion table and a logic circuitry therefor must be arranged in each node. The addition of such extra components complicates the network hardware in structure and causes an excessive increase in cost.
Still another problem of the prior-art ATM network with the "nodes dependent VCI/VPI uniformity" feature is that the system expandability and flexibility remain poor. More specifically, if a terminal is moved from a switch node presently connected therewith to another node in the ATM-LAN (such terminal movement may caused when a workstation computer is moved to another office within the same building), the VCI/VPI values assigned thereto become meaningless in a new destination after the movement was made. Simultaneously, the VCI/VPI values that have been owned by the destination node also become meaningless. It should be required that these VCI/VPI parameters are manually reassigned with different values by negotiation with the network due to the movement of the terminal. When viewed from the network side, this may result in a penalty which greatly decrease the expandability and flexibility inherent to the ATM.